Technical Field
The present invention relates to a heat insulating material including a porous sintered body of MgAl2O4 and having excellent heat insulating performance at a high temperature of 1000° C. or higher.
Description of the Related Art
The heat insulating material is required to have low thermal conductivity. A fiber-based heat insulating material such as glass fiber, ceramics having a low bulk density, such as a ceramic porous body, or the like is generally used. A heat transfer factor having an influence on the thermal conductivity can be considered by dividing the heat transfer factor into solid heat transfer, gas heat transfer, and radiation heat transfer.
For example, JP 2009-299893 A describes a fiber-based heat insulating material including a fiber material filled with aerogel. In the fiber-based heat insulating material, a heat insulating layer including an infrared reflection agent is coated with a porous coating layer to suppress the radiation heat transfer.
However, such a heat insulating material mainly contains silica aerogel, has low heat resistance, and has unknown thermal conductivity at a high temperature of 400° C. or higher.
On the other hand, in the ceramic porous body, the solid heat transfer is suppressed and the thermal conductivity is reduced by making the porosity of the ceramic porous body high.
However, an influence by the radiation heat transfer is large at a high temperature of 400° C. or higher. Therefore, in the heat insulating material to be used in such a high temperature region, the radiation heat transfer has been suppressed by adding a material having a high radiation rate, such as a metal oxide of zirconia, titania, or the like, and silicon carbide.
Furthermore, the present inventors have proposed that a spinel ceramic porous body having a predetermined pore-diameter distribution can suppress the solid heat transfer and the radiation heat transfer and can be used as a heat insulating material having excellent heat resistance at a high temperature of 1000° C. or higher (for example, refer to JP 2012-229139 A and JP 2013-209278 A).
JP 2012-229139 A or JP 2013-209278 A discloses, for example, that the spinel ceramic porous body having a predetermined pore-diameter distribution can suppress conductive heat transfer and the radiation heat transfer and can be thereby used as the heat insulating material having excellent heat resistance at a high temperature of 1000° C. or higher.
The spinel ceramic porous body described in JP 2012-229139 A or JP 2013-209278 A has heat resistance at a temperature of 1000° C. or higher which is higher than that in the related art. However, the spinel ceramic porous body described in JP 2012-229139 A or JP 2013-209278 A has a heat-resistant temperature of 1600° C. at most and compressive strength of about 0.8 MPa as long as disclosed.
Recently, a heat insulating material having higher performance has been needed. A heat insulating material having heat resistance, high strength, low thermal conductivity, and the heat insulating performance maintained even at a higher temperature of about 1800° C. has been desired. That is, for example, even a heat insulating material having excellent heat resistance and low thermal conductivity may be required to have more strength.